Electrical voltage sensors measure AC and/or DC voltage levels. They receive voltage inputs and provide outputs as analog voltage signals, analog current levels, switches, or audible signals. They can also provide frequency and modulated frequency outputs. For example, some electrical voltage sensors produce sine waves or pulse trains. Others provide amplitude modulation (AM), frequency modulation (FM), or pulse width modulation (PWM). Self-powered devices are commonly available. Handheld products are portable and do not require special mounting hardware. Electrical voltage sensors that mount on DIN rails and printed circuit boards (PCB) are also used. Through hole technology (THT) mounts components on a printed circuit board by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board.